What is Event horizon: Definition and 299 Discussions

In astrophysics, an event horizon is a boundary beyond which events cannot affect an observer. The term was coined by Wolfgang Rindler.In 1784, John Michell proposed that in the vicinity of compact massive objects, gravity can be strong enough that even light cannot escape. At that time, the Newtonian theory of gravitation and the so-called corpuscular theory of light were dominant. In these theories, if the escape velocity of an object exceeds the speed of light, then light originating inside or from it can escape temporarily but will return. In 1958, David Finkelstein used General Relativity to introduce a stricter definition of a local black hole event horizon as a boundary beyond which events of any kind cannot affect an outside observer. This led to information and firewall paradoxes, which encouraged the re-examination of the concept of local event horizons and the notion of black holes. Several theories were subsequently developed, some with, and some without, event horizons. Stephen Hawking, who was one of the leading developers of theories to describe black holes, suggested that an apparent horizon should be used instead of an event horizon, saying "gravitational collapse produces apparent horizons but no event horizons". He eventually concluded that "the absence of event horizons means that there are no black holes – in the sense of regimes from which light can't escape to infinity."Any object that approaches the horizon from the observer's side appears to slow down and never quite crosses the horizon. Due to gravitational redshift, its image reddens over time as the object moves away from the observer.In an expanding universe the speed of expansion reaches and even exceeds the speed of light, which prevents signals from travelling to some regions. A cosmic event horizon is a real event horizon because it affects all kinds of signals, including gravitational waves which travel at the speed of light.
More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Other distinct types include the Cauchy and Killing horizons; the photon spheres and ergospheres of the Kerr solution; particle and cosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.

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  1. Somoth Ergai

    B Entering a black hole -- I know I must be wrong (Help me understand why)

    From what I understand about time dilation and the relativity of simultaneity; if we imagine two people near a black hole and one of them begins to approach the black hole on a trajectory that crosses the event horizon. The stationary observer will never see the moving observer enter the black...
  2. H

    I Seeing history when crossing event horizon

    Looking at Kruskal diagram, it appears that light from all previous events of something crossing event horizon at that same point, reaches the falling observer when it crosses the event horizon. Is my interpretation correct?
  3. P

    B Is crossing a black hole's event horizon possible?

    As I understand things if you're hanging out in your space suit some distance away from a black holes event horizon and your buddy decides to dive on in you will never see him cross the event horizon. You'll see him approach the event horizon but never cross it. It would seem the time needed to...
  4. H

    B Can we see 2 black holes orbiting each other?

    as I understand time stops at the event horizon of a black hole for the far away observer; so can we actually see 2 black holes orbiting eachother? I also understood that the singularity is a moment in future. so what happens when 2 black holes orbit eachother with the singularites? from the...
  5. Christian Thom

    I Tidal effects on event horizon of binary rotating BHs

    The presence of a big mass (BH or neutron star) in the vicinity of a BH must have an effect on the shape of the event horizon, an indentation comes logically to mind. When the system is rotating, it would lead to tidal effects on the horizon that would disclose internal material of the BH. This...
  6. Bosko

    I A mirror behind the event horizon

    Thought experiment: What will happen if you place a mirror behind the event horizon and turn on the light in its direction?
  7. M

    B "If a black hole has more mass then the event horizon shrinks" ???

    The title is a direct quote of this video by Dr. Becky Smethurst, an astrophysicist specializing in black hole research. This is a mistake, right? Supermassive black holes, for example, don't have tiny radii, compared to stellar mass BHs. Then there's the equation she presents seconds later...
  8. Wo Wala Moiz

    I Do any real macroscopic black holes have a singularity?

    Here's my reasoning. The event horizon is the point where the escape velocity becomes greater than the speed of light. This results in the event horizon spacetime boundary having infinite time dilation. So, that must mean that inside the boundary of the event horizon, time dilation must...
  9. S

    I Entropy reversal in an infinite static universe?

    As far as I know, entropy could be reversed by the Poincaré recurrence theorem if it had a finite horizon given by some amount of vacuum energy causing an accelerating expansion. However, I found this lecture by Leonard Susskind () where he tells a way through which the vacuum could decay into...
  10. jaketodd

    I Event Horizon movie, with some interesting concepts (papers cited)

    Please hear me out; I've referenced some good papers below, and I think you'll find the movie clip intriguing. Is there anything solid to this movie? The faster than light travel. The dimensional gateway. The folding of spacetime, passing through the hole, and then the return of spacetime to...
  11. jaketodd

    Movies and/or music that got you interested in physics

    For me, Event Horizon was very scary but had enough sci-fi and sci-real to it that it captivated me.
  12. .Scott

    B Crossing the Bekenstein Bound at Black Hole Event Horizon

    The Bekenstein Bound places a upper limit on the amount of entropy that a given volume of space may contain. This limit was described by Jacob Bekenstein who tied it quite closely to the Black Hole Event Horizon. Put simply, black holes hold the maximum entropy allowed for their volume. If you...
  13. S

    B Does Gravitation Start at Black Hole Event Horizon?

    Has anyone done this calculation? It seems to me that if light and time disappear at the event horizon, it's a gravitation limit as well.
  14. Feynstein100

    I Black Holes: Event Horizon Causally Disconnected?

    I've heard quite frequently that events inside the event horizon of a black hole are causally disconnected from the rest of the universe. I take it to mean that while outside events can interact with the events inside of the horizon, the reverse is not true i.e. inside events cannot interact...
  15. B

    I Observing a Collapsing Shell: Time Dilation Explained

    What does and observer inside of a collapsing shell observe? Lets say we have a shell of matter collapsing to a black hole. What would observers near the center see? How would the rest of the universe appear when, The shell is approaching the Schwarzschild radius? After the shell passes the...
  16. J

    I Cosmic Event Horizon – Does it have any real effects?

    Using LightCone8 Cosmological Calculator and PLANCK(2018+BAO) data as input, we can get the following result: In the figure, Dhor is the event horizon and Dpar is the radius of the observable universe. Currently (t = 13.79 Gyr) Dhor has a value of 16.58 Gly. Does the event horizon have any...
  17. S

    I How many apparent horizons could the Universe have?

    I was reading a paper written by George Smoot [1], which assumes the holographic principle as true and conjectures that our universe would be encoded on the "surface" of an apparent horizon as the weighted average of all possible histories. In that way, there would be one world (or universe)...
  18. Jay B

    I Equation - cosmic event horizon

    Hi Everyone, I'm hoping someone can share an equation that would give the distance of the cosmic event horizon for a given time after the big bang. Thanks for any help! Jay
  19. K

    I What Happens Inside BH Event Horizon?

    I have read about the spaghettification of objects due to tidal forces as they get close to the singularity. Gravity at your feet is stronger than at your head, so you get stretched and pulled apart. In this case, the singularity is a point in space. But I also read about the time coordinate...
  20. A

    I Differences b/w Schwarzschild Radius & Event Horizon

    I understood that the event horizon is a null surface and not a place in space, what is the relationship between it and the Schwarzschild radius? Also, what does the Schwarzschild radius physically represent for example for an object such as a star?
  21. A

    B Black Hole Event Horizon: Atom Effects & Wave Detection

    Are the atoms of objects ripped off when they cross the event horizon? Does a metal rod that partially crosses the event horizon maintain its lattice structure of atoms? If I put a sound wave generator on the end of the bar that has crossed the event horizon how far can the waves be detected?
  22. U

    Finding Event Horizon & Ergosphere: Derivations & Formulas

    Homework Statement:: See below. Relevant Equations:: See below. I am trying to calculate the event horizon and ergosphere of the Kerr metric. However, I could not seem to find a proper derivation or formula to calculate the event horizon and ergosphere. Could someone point me to the...
  23. R

    I Black Hole Waterfall Analogy & Light Speed

    Recently I have seen a number of General Relativity visualisations that show spacetime flowing towards any mass, similar to water flowing into a sink hole. ScienceClic's video is an example. That model is also used in the "waterfall model" to explain the event horizon of a black hole, as the...
  24. ergospherical

    I Quasi-Static Change of Event Horizon Area

    Let ##\mathscr{H}## be a constant-##v## cross-section of the event horizon (area ##A##). The expansion is the fractional rate of change of the surface element, ##\theta = \frac{1}{\delta S} \frac{d(\delta S)}{dv}##. The problem asks to prove the formula ##\frac{dA}{dv} = \frac{8\pi}{\kappa}...
  25. S

    I Time Dilation at Moving Black Hole Event Horizon

    Hello everyone, I have a hard time to conceptualize the case of a moving black hole. We know from SR that time slows down for moving objects; but time dilation at the event horizon is already equal (tends) to zero. It seems that it can create some sort of conflict for the black hole movement...
  26. ergospherical

    Math Treatment of Approaching Event Horizon

    [This thread can be considered the A-level footnote to https://www.physicsforums.com/threads/is-there-an-inside-to-a-black-hole.1007588/] For a static [admits a hypersurface orthogonal timelike Killing field ##k##], spherically symmetric spacetime, a time coordinate ##t## can be chosen as the...
  27. Roberto Pavani

    B Exploring Time and Space Dilatation Near Black Hole Event Horizon

    As closer the observer will be to the event horizon, the more the time dilatation will be. As we know, if the observer O1 has a clock, another observer O2 very far from the black hole will se the O1 clock "slowing" down as O1 approach the event horizon. The limit is that the O1 clock "stops" at...
  28. A

    I Crossing Event Horizon: Physics of Disintegration?

    As you fall through the event horizon, for a time one part of your body is inside the event horizon and the other part is outside. At that point the two parts of your body are casually disconnected. So shouldn't it be severing the chemical bonds holding your body together at the point of the...
  29. M

    I Photon behaviour across EH threshold

    Physics is not my area of expertise. That being said, philosophy of science is, but I'm not here to discuss philosophy. I recently found myself trying to imagine how light behaves once it crosses the event horizon of a black hole. Presumably, between the event horizon and the singularity...
  30. S

    B Event horizon for observer within main horizon

    The event horizon of a black hole is defined with respect to observers far away, and we know that light from within the horizon can't reach a distant observer. But what if an observer is within the "main" event horizon? Presumably, there will be another horizon nearer to the center, such that...
  31. A

    I Dissolving Event Horizon w/Charged & Rotating BH

    I saw a fascinating video from PBS space time about dissolving an event horizon. See here for reference: The video addresses rotating kerr black holes and charged black holes, but doesn't talk about the combination of rotation and charge. So what happens when you spin up the black hole as...
  32. BWV

    B Star at black hole event horizon

    Curious if the time dilation at the edge of an event horizon would have the apparent effect of prolonging the life of the star to an outside observer - so for example a blue hyper giant at the edge of an event horizon with an expected main sequence time of, say, 500 million years, would remain...
  33. P

    B Can it escape the event horizon if the wavelength is long enough?

    Can electromagnetic radiation escape from the event horizon of a Black Hole if the wavelength is long enough? What if a Black Hole contains electric charge, hypothetically supposing we dumped a large number of protons into it? Electric charge is mediated by the electromagnetic force. So the...
  34. Y

    I Black Hole Event Horizon Hypothesis

    Main Question or Discussion Point Wouldn't the definition of the event horizon of a black hole be the radius at which the acceleration of gravity exceeds the speed of light, instead of the radius at which the escape velocity exceeds the speed of light?It's very clear to me that a...
  35. DarkMattrHole

    I Event Horizon Radius: Is it Constant?

    Let's say there is a black hole billions of miles away from earth, a hefty one such that a careless traveler could end up inside the horizon before noticing he'd been swallowed by the BH. Based on Earth observations the BH event horizon radius is r. We hop in a ship and go to a safe escape...
  36. arkantos

    I Who Will Win the Race at the Event Horizon?

    Hi guys. Imagine that in the exact instant when a massive particle A crosses the event horizon of a black hole, a Photon does the same,so that they have a race toward the singularity. Who will win the race? Will they have still different velocities?
  37. Chiron69

    I To collect all info at the event horizon

    Hi there! I have a question for anyone; If we could have built a shielded spaceship that can withstand all the radiation etc from a supermassive black hole. And we managed to park at the event horizon. And we want to collect all the visual data that's there in laters, how could we get all...
  38. R

    I Gravity at the Event Horizon of a Black Hole

    Acceleration due to gravity at the Event Horizon of a Black Hole.
  39. M

    I Event Horizon Gravity: Infinite or Not?

    In my browsing around various science forums a have come across the comment that the gravity field becomes infinite at the event horizon. I have always thought that this is a misunderstanding, and that it only becomes infinite at the central singularity. Then I found this same statement in...
  40. DennisN

    I An observer passing the event horizon of a black hole (Part 2)

    Hi all, this is a new scenario I got thinking about after having received great feedback and corrections from other PF'ers in this thread. Thanks again for the great help! This new scenario is similar to the previous one, but with a twist including a mirror. And as I said in that thread, I am...
  41. DennisN

    I An observer passing the event horizon of a black hole

    Hi all, I've just read this entire thread and watched the videos about black holes posted by @PeroK, which I liked very much (thanks @PeroK! :smile:). I am not particularly well aquainted with GR and my questions are concerning the often mentioned statement that an observer that passes the...
  42. A

    B Kinetic Energy of an Object Falling Into an Event Horizon

    Hi, When objects fall in a gravitational field, they convert gravitational potential energy into kinetic energy. Because energy is always conserved: amount of kinetic energy gained = amount of gravitational potential energy lost. Now the gravitational energy lost should be equal to the amount...
  43. W

    B Can we see matter crossing the Event Horizon with a powerful telescope?

    There is some confusion on my part what the actual reality is at the Event Horizon, since there appears to be different answers in using Kruskal-Szerrkes coordinates or Schwartzschild coordinates. Reality does not have two answers. There is only one right one. Asked multiple times on this...
  44. Staticboson

    B Physics near the event horizon

    Understanding that I might be pushing the limits into heuristic territory, I'm wondering how much agreement exists on whether the theory holds up in the proximity of an event horizon. This came up during a recent discussion about matter falling into the black hole and the Schwarzschild solution...
  45. A

    B A little logical doubt on Hawking radiation

    Summary: As hawking radiation is based on quantum fluctuations, can they cancel out each other due to equal probabilities of a particle remaining in or drifting away? I recently learned how hawking radiation actually works. It is based on quantum fluctuations which happen randomly in space...
  46. mfb

    Stargazing Event Horizon Telescope Results Released Yesterday (April 10, 2019)

    Media Advisory: Press Conference on First Result from the Event Horizon Telescope April 10, 15:00 CEST (13:00 UTC. In 8 days and 13 hours) Livestream links are on that website. The Event Horizon Telescope is a collection of radio telescopes all over the world which recorded data from the...
  47. G

    B 83 quasars right up against the Event Horizon

    A few years ago I became intrigued by articles reporting the discovery of stars very close to the purported Big Bang; 400 million years seems an awful short time for a star to evolve. Then more recently the discovery of 2nd generation - hydrogen, carbon stars - in the same proximity, supposedly...
  48. S

    A Penrose diagram of black hole with a changing event horizon

    Dear all, I have a question on Penrose diagrams. Consider a collapsing star that forms a black hole with a Schwarzschild radius normalized to 1. What happens in the Penrose diagram when additional matter falls in? I suspect the diagram then has to look like this : When the outer shell (second...
  49. D

    B Can event horizons cross for passing black holes at high speed

    Scenario: You have two black holes approaching, one from the left (A), one from the right (B), each at speed S. They are offset vertically. S is sufficiently high that they will deflect passed each other without merging. Question: Suppose the speed S is high enough so that the event...